So-called sandwich panels have been typically used for various purposes, including transport machines such as automobiles and aircraft; building materials; floor boards for beds; electric device housings; and sports and leisure activities. A sandwich panel includes two skin material sheets and a core material interposed between the skin material sheets. A laminated structure of the skin material sheet, the core material, and the skin material sheet is the basic form. In accordance with the purpose, the functions required of the sandwich panel may vary. For example, when visual aesthetics are considered important while not much strength is required, such as in the case of an interior panel for use in the bathroom, a decorative material may be additionally affixed to the front-side skin material sheet presenting the exterior appearance. In this way, importance is given to the surface property of the decorative material or the overall molded shape. When the use is as structural material, strength is demanded more than visual aesthetics. In this respect, interior material components for transport machines such as automobiles and aircraft, particularly the cargo floor board, deck board, rear parcel shelf and the like for vehicles, are required to be lightweight from the viewpoint of increasing fuel economy, as well as being strong. Accordingly, resin sandwich panels including the skin material sheets and core material that are made of resin have often been used.
More specifically, when a sandwich panel made of resin is utilized as an automobile cargo floor lid, the sandwich panel is used for the placement of heavy baggage on the cargo floor lid. Accordingly, the sandwich panel is required to have not just visual aesthetics but also the stiffness (particularly bending stiffness) to withstand the weight of the baggage. On the other hand, being lightweight is also demanded from the viewpoint of increasing fuel economy. Therefore, it is necessary to overcome the technical problem of achieving both high stiffness and lightweight, which is difficult to solve. Accordingly, as skin material sheets of the resin sandwich panels for such uses, hard resin material having a high Young's modulus has been adopted. Meanwhile, with regard to the core material, the bulk (core material thickness) may be increased so as to maximize the interval between the skin material sheets, so that the core material per se can be made lightweight while achieving an increase in section modulus. For this purpose, foamed material is adopted, for example. Alternatively, a core material with an internal hollow portion or a number of recesses in the surface thereof may be adopted.
In such resin sandwich panel, in order to achieve weight reduction by providing the core material with voids, resin may be foamed to produce numerous internal cells, as disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-334801 and 2008-247003. Japanese Unexamined Patent Application Publication Nos. 2006-334801 and 2008-247003 commonly relate to the sandwich structural body for interior material panels of automobiles, such as deck boards and floor panels. According to Japanese Unexamined Patent Application Publication No. 2006-334801, two strips of molten parisons are positioned between split mold blocks. Between the two strips of molten parisons, a pre-molded core material 13 made of resin, such as polypropylene, is disposed. Then, molding is performed by pressing the split mold blocks using vacuum or compressed air. According to Japanese Unexamined Patent Application Publication No. 2008-247003, with the split mold blocks disposed one above the other, a back-side sheet material that has been previously molded and then softened by re-heating is placed on the lower mold block. Then, the back-side sheet material is vacuum-suctioned via the lower mold block, whereby the back-side sheet material is shaped. A foamed resin of polyethylene, for example, is placed on the back-side sheet material, with a front-side sheet material that has been previously molded and then softened by re-heating placed on the foamed resin. Thereafter, the mold blocks are clamped while performing vacuum suction via the upper and lower mold blocks, whereby a sandwich structural body is molded. With the sandwich panel thus molded, weight reduction can be achieved by the formation of cells by foaming.
However, when the surface of the resin core material 13 and the opposing thermoplastic resin sheet are welded, air is present therebetween. Accordingly, one or a plurality of air accumulations of various sizes may be formed at unexpected positions. As a result, the following technical problems may be caused.
First, molding failure or lack of stiffness may be caused in the resin sandwich panel. More specifically, unexpected air accumulations are formed between the surface of the resin core material 13 and an opposing surface of the corresponding thermoplastic resin sheet. Accordingly, the air accumulations have a two-dimensional expanse. However, their thickness is very small, and their volume is also small. When the split mold blocks are clamped so as to sandwich the resin core material 13, the hermetic space inside the mold is pressurized. As a result, if there are air accumulations between the surface of the resin core material 13 and the opposing thermoplastic resin sheet, an internal pressure increase is readily caused by the clamping, resulting in bulging and an increase in the air accumulation region. This may cause peeling between the surface of the resin core material 13 and the opposing thermoplastic resin sheet. As a result, weld peel-off is readily caused, triggering a molding failure. Even when molding failure is not caused, insufficient welding may cause a decrease in stiffness of the resin sandwich panel as a whole. Secondly, the aesthetic exterior appearance of the resin sandwich panel may be lost. More specifically, if the air accumulation region expands as the split mold blocks are clamped as described above, portions of the outer surface of the thermoplastic resin sheet corresponding to the air accumulation region may partially bulge. As a result, the flatness of the outer surface of the thermoplastic resin sheet is lost, whereby the aesthetic exterior appearance of the resin sandwich panel is compromised. It is technically difficult to perform clamping of the split mold blocks while eliminating such air accumulations in advance.
Third, with the sandwich panel molding method, it may be difficult to obtain a high-quality sandwich panel. More specifically, for forming cells by foaming, as in Japanese Unexamined Patent Application Publication Nos. 2006-334801 and 2008-247003, chemical foaming technology or physical foaming technology is generally used. In either case, the foaming by itself does not pose a restriction on the core material molding method. As in the case of non-foamed core material, extrusion molding, injection molding, blow molding, or press molding may be utilized. However, as the foam expansion ratio is increased, it becomes difficult to adjust the foam expansion ratio, particularly in the case of extrusion molding or injection molding. As a result, it becomes difficult to obtain desired quality.